1. Field of the Invention
The present invention relates to a power supply system for supplying d.c. power from a center equipment (e.g., a telephone exchange) to a terminal device by way of a two-wire line.
2. Description of the Prior Art
A known type of system for supplying d.c. power from a center equipment, e.g., a telephone exchange to a terminal device by way of a two-wire line will be described first with reference to the accompanying drawings.
FIG. 1 is a wiring diagram showing by way of example the construction of a prior art power supply system.
In the Figure, numeral 10 designates a terminal device, 11 a hook switch of the terminal device, 12 a terminating power supply rectifier of the terminal device, 13 a power supply circuit for converting and providing the required d.c. power to the respective component parts of the terminal device 10, 20 a two-wire line, 30A a terminal circuit of the center equipment (e.g., a telephone exchange), 31 a d.c. power source of the circuit 30A, 32 a polarity-reversing switch, and 33 a terminating signal input terminal.
Where the terminal device 10 is an originating party, its hook switch 11 is turned ON (closed) so that the corresponding terminal circuit 30A of the center equipment applies the positive and negative potentials of the d.c. power source 31 to the power supply circuit 13 by way for example of the lines A and B of the two-wire line 20 and the required d.c. power is supplied through conversion to the respective parts, thereby effecting the desired communication between the terminal device 10 and the center equipment.
In this case, a terminating signal TER is not applied to the terminating signal input terminal 33 of the terminal circuit 30A from another part of the center equipment (e.g., a telephone exchange controller) so that the polarity-reversing switch 32 is not operated, and the positive and negative potentials of the d.c. power source 31 are connected as such to the lines A and B of the two-wire line 20 thereby supplying the required power to the power supply circuit 13.
Where the terminal device 10 is a terminating party, a terminating signal TER is applied to the terminal interfacing circuit 30A via the terminating signal input terminal 33 from another part of the center equipment (e.g., the telephone exchange controller) and the polarity-reversing switch 32 changes its state. In other words, the positive and negative potentials of the d.c. power source 31 are reversed and connected to the lines A and B of the two-wire line 20.
As a result, since the rectifier 12 of the terminal device 10 is in the forward direction irrespective of whether the hook switch 11 is ON, the required power is supplied to the power supply circuit 13 so that the respective parts of the terminal device 10 are brought into operation and a given signal is received from the center equipment thereby effecting the desired communication therebetween.
Also it is arranged so that in the non-communicating condition where there is no originating communication or terminating communication, the hook switch 11 is turned OFF (open) and the polarity-reversing switch 32 does not change state with the rectifier 12 being placed in the reverse direction, thereby preventing the supply of power to the terminal device 10 from the d.c. power source 31.
With this prior art system, if, for example, the lines A and B of the two-wire line 20 were connected in wrong polarity during the initial installation of the terminal device 10, the power would be supplied wastefully during the non-communicating period, whereas when the terminal is terminated so that the polarity-reversing switch 32 performs a polarity reversing operation in response to the terminating signal TER, the supply of power becomes impossible.
It is to be noted that if the two-wire line 20 must be connected after the polarities have been investigated and confirmed satisfactorily and thus, the connection requires much trouble and is complicated extremely.